Shaders are looking for textures and since one isn't bound for the preview block then it appears invisible.

In PreviewRenderer.java,
Replace the GL11 functions with :
GL11.glEnable(GL11.GL_BLEND);
mc.renderEngine.bindTexture(TextureMap.locationBlocksTexture);

This will show a ghost block with shaders enabled so the player can at least see where the block will be placed.

This quick fix allows the user to also see the preview block with shaders disabled.

I am not well versed in obsolete ogl. Maybe you can improve this to look better.

I noticed this error, the box sometime get invisible if you use a shader, maybe this fixes this bug i will try it out.

I'm now trying to fix it: sp614x/optifine#663

Looks like this issue has been solved somehow (at least for 1.11.2)